A model for studying oxygen transport was used to examine the acute blood flow autoregulation phenomenon and its effectiveness for maintaining tissue oxygenation under different physiological conditions. The effectiveness of tissue oxygenation was judged by the level of venous blood oxygen tension, PVo2. Three sets of conditions were studied with, and again without, autoregulation present. First, arterial blood oxygen tension, Pao2, was lowered in steps while changes in PVo2 and blood flow were recorded. Second, blood hemoglobin concentration was lowered in steps while Pao2 remained normal. Finally, the rate of oxygen consumption was increased, simulating the increase in metabolism that occurs during strenuous exercise. Although the basic negative feedback gain had been set at the same level in each case, in comparing the results of the different simulations it became apparent that there were notable differences in each of the autoregulatory responses. These observations indicated that the overall effectiveness and the total feedback response of autoregulation could be highly variable, depending on the conditions.